In the UK the baby pigeons/doves that you are most likely to come across usually due to nest destruction or nest predation - are the wood pigeon, the feral pigeon , the collared dove and the stock dove. However, baby feral pigeons often need rescuing when their nests are destroyed by pest controllers or property owners . This is what each of those look like :

Baby wood pigeon: even at this age you can see the first white feathers which will form his white wing band sprouting at the bottom edge of his wing. The white neck ring won't appear until it is an adult.